I have set an email alias into my company domain's Gmail account. Recently (I don't know exactly when it stopped working) people started getting a "no such user" message from Google when using that alias.

At last, I have found the aliases at user profile. Aliases for both of my domains are displayed but only for my main domain name the aliases are removable (Eltávolítás means Remove), so there is a problem with my secondary domain, buday-rd.hu.